1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and in particular, to an electrophotographic image forming apparatus for transferring a toner image formed on a surface of a latent image carrier to a recording medium using dual-component developer to output and an image forming method.
2. Description of the Prior Art
In a conventional electrophotographic image forming apparatus, generally, a surface of a latent image carrier (such as a photoreceptor drum) is charged, an electrostatic latent image is formed by exposing the image to the charged region, the electrostatic latent image is visualized (developed) by a developing device as a toner image, the visualized toner image is electrostatically transferred to a recording material such as recording paper, and thereafter the toner image transferred to the recording material is fixed to the recording material by a fixing device.
This type of image forming apparatus has employed a method, in which when dual-component developer including toner, and a magnetic particle and carrier for applying an electric charge to toner is used to perform development, only toner among toner and carrier in developer is adhered to a latent image carrier to be consumed.
Hence, in the image forming apparatus, based on the measurement result of a toner concentration sensor for detecting a toner concentration of developer, the toner concentration control is generally performed so as to keep the toner concentration of the developer constant.
Moreover, it is generally called process control that process conditions such as a charging potential, an exposure amount, and a developing potential are adjusted in response to an environment of an apparatus and the passage of time to obtain a constant-quality image at all times.
As the conventional technology of the process control, a technology is disclosed that in an image forming apparatus, a surface potential of a photoreceptor drum is measured, a toner adherent amount adhering to the photoreceptor drum is predicted based on image data, and the toner adherent amount actually adhering to the photoreceptor drum is measured, to thereby change at least one of image forming conditions including a charging amount, a developing bias voltage, an exposure intensify, and a toner concentration based on the comparison result and adjust the process conditions (for example, refer to Patent Literature 1: Japanese Patent Application Laid-Open Hei 5 No. 2306).
Further, a technology is disclosed that in an image forming apparatus, based on a deviation from a predetermined target value of a measured value for an image density of a reference toner image (hereinafter referred to as “a toner patch”), and conversion coefficients each set correspondingly for a case where the measured value is within a reference density area, a case where the value is within a low density area lower than the reference density area, or a case where the value is within a high density area higher than the reference density area, a toner concentration of developer is controlled (for example, refer to Patent Literature 2: Japanese Patent Application Laid-Open No. 2002-196552).
However, for example, even development condition in which the same developing potential is secured has greatly different development characteristics and printing characteristics depending on the previous print jobs. That is, when low-print jobs are repeated during the process control, decrease in the image density occurs due to roughness in a solid part. This is because stress on developer caused by that turnover of toner is less worsens the fluidity. On the contrary, when high-print jobs continue, developer charging amount is reduced because turnover of toner is much, resulting that the toner adherent amount is increased to cause variation of the image density.
Further, the Patent Literature 1 describes that a toner image density is changed by simply controlling a toner concentration, but does not give consideration to the previous print jobs, thus posing a problem that the image quality is greatly deteriorated in some printing conditions.
Further, in the Patent Literature 2, since a toner concentration is controlled based on the result of only an image density of a toner patch, the surface potential at that time is unclear and merely a developer concentration is secured, thus, similarly to the case of the Patent Literature 1, posing a problem that the image quality is greatly deteriorated in some printing conditions.
Accordingly, it is desired to maintain a predetermined image quality level while considering the environment of an image forming apparatus and the passage of time.